1. Field of the Invention
This invention relates to a method and apparatus for locating gas leaks from underground gas-containing pipelines by acoustic means. More particularly, this invention relates to the use of acoustic sensors for pinpointing the location of a gas leak from an underground gas pipeline so as to avoid unnecessary excavation of the ground surrounding the pipeline.
2. Description of Related Art
Gas leaks from underground gas pipelines are a significant problem in terms of both the amount of revenues lost to gas companies and the potential hazards presented. The locations of such leaks are also very difficult to pinpoint, particularly when the gas has migrated beneath pavement or frozen ground, when the gas has spread over a large area, when multiple leaks are present, or when the water table is high and gas and air are displaced from the ground. Leaks from low-pressure (6-12 inches of water column; xc2xc to xc2xd psig) cast-iron pipelines are often particularly difficult to pinpoint using current technology, such as a combustible gas indicator. Given the substantial costs associated with excavation of the ground surrounding a leaking underground gas pipeline, it is critical that the locations of such leaks be readily determinable to minimize the extent of the excavations required, to avoid unnecessary excavations and to limit the amount of manpower and time required to locate the leaks. The objective of leak detection methods is to be able to pinpoint the location of a leak within plus or minus two feet, which corresponds to the size to the hole typically required to repair the leak. Newly emerging keyhole leak repair methods put a premium on high accuracy (xc2x14 inches) leak pinpointing. It is also highly desirable that any such leak detection methods and devices employing such methods be readily usable in the field by gas utility crews.
One known technique for detecting leaks from underground gas pipelines involves detecting the sound/vibrations created by the gas escaping through the leak. However, to accomplish this feat, it is necessary to be able to detect very small leak acoustic signals and to be able to differentiate the leak sounds/vibrations from ever present background noises.
Sonic leak detectors and leak pinpointers can be divided into two general groups: detectors for leaks of liquid materials and detectors for leaks of gaseous materials. Liquid leaks generally create louder sound waves than gas leaks and higher pressure leaks are generally louder than lower pressure leaks. U.S. Pat. No. 4,083,229 to Anway teaches location of a leak in an underground water pipe using spaced detectors in contact with the pipe to improve sensitivity. U.S. Pat. No. 3,223,194 to Michael teaches a sonic vibrator locator for detecting leaks in water pipes, which locator has multiple vibration transmitting prongs in a specially shaped base and in a special arrangement relative to a transducer, which may be used for detecting vibrations in soft top earth.
With respect to leak detection in connection with gas pipelines, U.S. Pat. No. 4,289,019 to Claytor teaches a method and apparatus for passive detection of a leak in a buried pipe in which two detectors of acoustic signals are placed at separate locations on opposite sides of the leak. The signals detected by the two detectors are coupled to a single location for processing by an apparatus for measuring the correlation between the two signals. The cross-correlogram of the two signals provides a measure of the distance of the leak from each of the two measuring points and, thus, the location of the leak.
U.S. Pat. No. 5,058,419 to Nordstrom et al. teaches a method and apparatus for determining the location of a sound source, which apparatus includes a first detector located at a first location remote from the sound source, a second detector located at a second location remote from the sound source, and a computer which receives signals from the two detectors. The computer calculates a first frequency component and a first phase angle of the first signal and a second frequency component and a second phase angle of the second signal. The computer then calculates phase differences between the first phase angle and the second phase angle at each of a plurality of frequencies and converts the phase differences into a plurality of time lags. The computer then adds to each time lag of the plurality of time lags integer multiples of an associated period to form a plurality of potential time delays at each of the plurality of frequencies. The computer then determines which time delay occurs most frequently among all of the plurality of potential time delays at all of the plurality of frequencies. Utilizing the most frequently occurring time delay, the computer then calculates the location of the sound source.
U.S. Pat. No. 5,531,099 to Russo teaches a method of determining a defect in a buried conduit in which sensors are attached to the conduit at two locations separated by a known distance and a vibration imposed upon the conduit on one side of the sensors, which vibration is detected by both sensors. The process is then repeated where a second vibration is imposed on the other side of the sensors. The signals generated by the sensors are recorded, in either analog or digital form, filtered to pass a frequency band from 4000 to 8500 Hz to discriminate against turbulent flow noise in the system, noise transmitted by the conduit, and single frequency tones, and then a cross-correlation function from data obtained from the sensors from the first imposed vibration calculated to obtain a raw plot of a first time differential, and the same is done for the second vibration. Each raw plot of time differential is smoothed to obtain a peak time differential in each plot. The center velocity of propagation is determined using the first peak time differential and the known spacing between the sensors, and the process is repeated for propagation in the other direction. The flow rate and direction of the medium in the conduit are then calculated from the difference in velocities.
U.S. Pat. No. 5,349,568 to Kupperman et al. teaches a method and system for locating fluid leaks in pipes in which a first microphone is installed within fluid in the pipe at a first selected location and sound is detected at the first location. A second microphone is installed within the fluid in the pipe at a second selected location and sound detected at the second location. A cross-correlation is identified between the detected sound at the two locations for identifying a leak location.
U.S. Pat. No. 4,858,462 to Coulter et al. teaches a method and apparatus for locating a leak in which the acoustic emissions generated by the leak are continuously monitored from at least two spaced locations using detectors at the spaced locations to form at least two continuous signals having background noise and spikes corresponding to the background noise and spikes of the acoustic emission, the spikes in the signals of the detectors being offset from each other in time by an amount corresponding to the difference in travel time for an acoustic emission to each of the spaced locations. To remove the background noise from the spikes in each signal, a detection threshold value is set in a floating manner so as to correspond to an average level for the signals. The offset in time between the spikes of the signals, in real time, is then measured to determine the relative position of the acoustic emissions with respect to the spaced locations.
U.S. Pat. No. 5,038,614 to Bseisu et al. teaches a method for determining the location of fluid leaks in conduits in which axial and/or torsional vibrations and strains on a conduit and/or fluid pressure fluctuations in the conduit are measured at spaced apart points and the frequency patterns analyzed and compared to determine the location of fluid leakage or the occurrence of some other event associated with operation of certain mechanisms, such as valves, interposed in or connected to the conduit.
U.S. Pat. No. 5,416,724 to Savic teaches an apparatus for determining the location of leaks in an underground pipe comprising a plurality of remote acoustic transmission sensor units distributed along the pipe, each unit containing equipment for analyzing acoustic signals from the pipe. The equipment includes a mechanism for identifying acoustic features of the acoustic signals which distinguish the acoustic signals of a leak from ambient acoustic signals and a control unit connected to each of the remote units for further analyzing the signals to determine the proximity of the signal to a particular remote unit and, using the amplitude of the signal and the transmission characteristics of the pipe, determining the location of the leak.
U.S. Pat. No. 5,117,676 to Chang teaches a leak detection system comprising a plurality of acoustic microphones disposed along the exterior surface of a pipeline and an acoustic spectrum analyzer responsive to signals from the microphones for detecting peaks in the spectral content of the signals which arc at wavelengths which are multiples of twice the wall thickness. The vicinity of the hole location can then be determined by locating the microphone which generated the signals having peaks at the specific wavelength or harmonic thereof.
Previous efforts to improve the sensitivity of sonic detectors and control extraneous noise have not been as successful as desired, particularly for gas leaks. Numerous attempts at addressing these issues have been made over the years, but with only limited success. U.S. Pat. No. 4,455,863 to Huebler et al. teaches a method and apparatus for locating gas leaks from underground pipelines using a sound transducer attached to an elongated probe inserted into the ground for a substantial portion of its length. The elongated probe and transducer combination has an effective mechanical resonant frequency equal to or below the electrical resonant frequency of the sound transducer. The apparatus and method are said to improve sensitivity for detection of sounds created by leaking gas, thereby providing more accurate pinpointing of the gas leak. However, implementation requires a typical sensor/probe spacing of about two feet, which is extremely limiting in terms of the length of pipe that can be evaluated over a given period of time. In addition, the apparatus and method are generally not sensitive enough to detect leaks from cast-iron joints, cannot sufficiently minimize acoustic noise from buried electrical cables, and are limited in their ability to minimize acoustic and air traffic noise as well as other noises typically encountered at leak sites.
Accordingly, it is one object of this invention to provide a method and apparatus for sonic location of gas leaks from underground gas pipelines having improved sensitivity over conventional sonic gas leak detection means.
It is one object of this invention to provide a method and apparatus for sonic location of gas leaks from underground gas pipelines which increases the efficiency of sonic leak detection over conventional means by increasing the length of pipeline that can be considered in a given period of time.
It is yet another object of this invention to provide a method and apparatus for sonic location of gas leaks from underground gas pipelines which enables the detection of gas leaks from cast-iron joints.
It is still a further object of this invention to provide a method and apparatus for sonic location of gas leaks from underground gas pipelines which is more effective at addressing issues related to background noise than conventional means.
It is another object of this invention to provide a method and apparatus for sonic location of gas leaks from underground gas pipelines which is sufficiently accurate to reduce the extent of excavation required to repair the leak compared to conventional means.
These and other objects of this invention are addressed by a method and apparatus for locating gas leaks from underground gas pipelines in which a first acoustic sensor having a first signal output is positioned in the ground substantially above the underground gas pipeline or at a distance from the underground gas pipeline. In the former case, it is important that the first acoustic sensor not be located directly above the leak. A second acoustic sensor having a second signal output is positioned in the ground at a plurality of locations substantially above and along the underground gas pipeline. The output signals from the first acoustic sensor and the second acoustic sensor are measured for each location of the second acoustic sensor. Adaptive filtering is applied to the output signals resulting in an adaptively filtered output signal from the second acoustic sensor corresponding to each of the locations of the second acoustic sensor, whereby signal components common to both output signals, which common components correspond to the background noise, are eliminated from the output signal from the second acoustic sensors. An rms voltage for the output signal from the first acoustic sensor and an rms voltage for each of the adaptively filtered output signals from the second acoustic sensor and the differences there between are determined. The location of the second acoustic sensor corresponding to the largest positive difference between the rms voltage of the first acoustic sensor and the rms voltage of the second acoustic sensor is then determined, which location corresponds to the location closest to a gas leak from the underground gas pipeline. In accordance with one preferred embodiment of this invention, in place of one second acoustic sensor disposed at multiple locations, a plurality of spaced apart second acoustic sensors disposed along a length of the underground gas pipeline are employed.
The acoustic sensors employed in the method of this invention may be accelerometers, velocity sensors or geophones or combinations thereof and are mounted on probes which are driven into the ground at the desired locations. The signal outputs from the sensors are operably connected to electronic means for measuring the output signals from the acoustic sensors, which electronic means are operably connected to electronic and signal processing means for reducing the effects of background noise on the measured output signals. The electronic and signal processing means employs a combination of at least two noise reduction techniques adaptive filtering and calculation of rms values over selected segments of the output signals.